An IF (interstitial free) steel with an ultra low carbon content and a low nitrogen content for the use of automobiles is generally produced in a manner described hereinbelow. Molten iron produced by melting using a blast furnace is subject to refining with oxygen blowing in a converter into a molten steel (hereinafter referred as “produced by melting and refining processing,” or simply as “refining-produced” or “refined”, for example). Then, the molten steel is subjected to secondary refining performed using an RH (Ruhrstahl-Hausen) degassing system, and then the molten steel is formed into a steel cast slab by using a continuous casting machine. Further, in regard to a steel sheet of, for example, DQ (drawing quality) and EDQC (extra deep drawing quality) for the use of, for example, automobiles and cans, the content of nitrogen (symbol: N) is required to be 35 ppm or less.
Conventionally, a technique for reducing the nitrogen content of a steel sheet such as described above is known. In the technique, a carbon (symbol: C) source, such as a carbon-containing material, is added in a converter into a molten iron, and then an oxygen containing gas is blown into to promote decarburization reaction, thereby to accomplish nitrogen removal of molten steel being obtained. More specifically, the nitrogen. (symbol: [N]) in-the molten steel is likely to be absorbed onto a bubble surface of the CO gas, so that nitrogen removal in the molten steel proceeds faster as the amount of the generated CO gas is greater. In this case, the nitrogen content of molten steel tapped from the converter is about 30 ppm. Further, when the molten steel is processed in an RH degassing system, further nitrogen removal is performed even by reducing pressure in accordance with Expression (1) shown below in addition to the nitrogen removal through the decarburization reaction caused by, for example, oxygen in the molten steel or blowing of the oxygen containing gas.2[N]=N2  (1)
More specifically, nitrogen removal proceeds as nitrogen in the molten steel is gasified at the interface of the gas and the molten steel.
In the processing steps for execution in a sequential arrangement of “blast furnace→converter,” large amounts of iron ore (sintered ore) and coke are used as main materials fed into the blast furnace. Hence, not only facilities, namely a sintering plant and a coke plant, are necessary, but also the facility costs of blast furnace and converter themselves are notably high. Further, in recent years, also the necessity of a process replacing that using the blast furnace has increased from the viewpoint of reduction of CO2 emission. Japan is a scrap exporting country. This is due to the fact that processes sufficiently utilizing scrap as an important iron source have not been developed in Japan. Conventionally, scrap has been used in electric furnaces and has been used for the manufacture of reinforced bar and special steel bars/wire rods products, for example. In addition, scrap has not been used by automakers for automotive steel sheets, so that the usage amount of scrap has not been increased so much.
However, when desired low-nitrogen steel can be refining-produced by use of only an electric furnace, not only the facility costs can be reduced, but also the steel can be manufactured with use of only iron scrap, but without use of iron ore as main material. As such, advantages such as use of inexpensive materials and reduction in the amount of CO2 emission can be expected. Of course, in the case of IF steels, such as steel sheets for automobiles and cans, attention is paid on not only for nitrogen, but also on impurity elements (such as Cu and Ni, for example) called “tramp elements.” Such tramp elements are less preventable from mixing into molten steel in the use of iron scrap-and that are not desirable for steel properties. However, it is also known that such tramp elements can be prevented into molten steel when an iron source, such as HBI (hard briquet iron), charcoal pig iron, or cold pig iron, is used in combination while paying attention on components of iron scrap intended to be used. As such, it is eagerly desired that the low-nitrogen steel is refining-produced by use of an electric furnace, but without using the processing steps for execution in the sequential arrangement of “blast furnace →converter.”
Nevertheless, however, it is now assumed that an attempt is made to refining-produce the IF steel through the series of the processing steps for execution in a sequential arrangement of “electric furnace→RH degassing system→continuous casting machine by use of inexpensive iron scrap as a main iron source. In this case, compared to the converter, not only the electric furnace as a vessel is inferior in the sealing performance (against the atmosphere), but also the usage rate therein of molten iron, which has a high carbon content, is low. Hence, even when an attempt is made to perform nitrogen removal through the decarburization reaction, is performed, the nitrogen content of the molten steel tapped is in the range of about 50 to about 100 ppm at lowest. Further, although an attempt is made to perform nitrogen removal in the molten steel through reducing pressure by use of the RH degassing system (see FIG. 3), nitrogen in the atmosphere is drawn thereinto due to leakage from a snorkel. Hence, even when the-process period of time is increased as desired, nitrogen removal does not proceed so much, and it is difficult to refining-produce a low-nitrogen steel having a nitrogen content of 30 ppm or less.
Then, in order to accomplish the nitrogen content reduction of molten steel in electric-furnace steelmaking, an approach is used in which nitrogen removal is caused in the manner that cold pig iron and molten iron having high carbon contents are increased to thereby activate the decarburization reaction. Further, the molten iron is tapped in a so-called “rimming status” in which also the oxygen amount in the molten steel is increased, thereby to reduce absorption of nitrogen into the molten steel occurring during tapping. Nevertheless, however, the nitrogen content of the molten steel obtainable in the approach is limited to a range of from 30 to 50 ppm. Further, since there occurs also the nitrogen absorption from the atmosphere, it was impossible to stably refining-produce molten steel having a nitrogen content of 40 ppm or less. As such, such reduction of the nitrogen content of molten steel has been given up, but there has been developed a technique of manufacturing steel excellent in formability in the manner that the nitrogen content is limited to a range of from 40 to 90 ppm, and other elements are regulated (see Japanese Patent Publication No. 3177146, for example).
Further, generally, in the case of primarily producing ferric stainless steel having a low-nitrogen content, a so-called “VOD process” is used. More specifically, a VOD (Vacuum Oxygen Decarburization) degassing system shown in FIG. 2 is employed to replace an RH degassing system, and reduction of the nitrogen content is accomplished through processing steps for execution in a sequential arrangement of “electric furnace→VOD degassing system→continuous casting machine.” According to the technique, the carbon content of the molten steel to be treated in VOD degassing system is previously increased, and as described above, nitrogen removal is caused through the decarburization reaction and also nitrogen removal under pressure-reducing is applied in combination, thereby to promote nitrogen removal.
Further, in regard to nitrogen removal in molten steel, various researches have hitherto been conducted. As a consequence, it has become clear that nitrogen removal occurs with Expression (2) representing a so-called “slag-metal reaction” between molten steel and slag and Expression (3) representing the slag-gas reaction that causes shifting from in-slag nitrogen ion.[N]+[Al]+3/2(O2−)=(N3−)+1/2(Al2O3)  (2)2(N3−)+3/2O2=N2+3(O2−)  (3)
In the above, each of those indicated with not parentheses nor brackets indicates a gas state, [ ] indicates a state where the item is contained in the molten steel, and ( ) indicates a state where the item is contained in the slag.
For example, a technique using an approach for accomplishing reduction of the nitrogen content in the following manner is known (see Japanese Unexamined Patent Application Publication No. 2005-232536 for example). In this approach, a so-called “VOD process” for the refining-production of the ferric stainless steel is applied to the refining-production of ordinary carbon steel. In this case, carbon and aluminum (symbol=Al) are added to a molten steel tapped from an electric furnace, and then the molten steel is transferred into a VOD degassing system for oxygen blowing, thereby to cause heating-up by Al agent. Thus, there is applied a condition causing the decarburization reaction in preference to oxidation of Cr contained in the molten steel, and nitrogen removal is performed in the manner that degassing effects resulting from the decarburization reaction is activated utilizing the heating-up by Al agent. In this technique, nitrogen removal is performed in accordance with Expressions (1) and (2).
Further, techniques using Expressions (2) and (3) include such a type as described herein (see Japanese Unexamined Patent Application Publication No. 8-246024). According to this technique, in a VOD system such as described above, while a molten steel is being deoxidized with Al being added thereto, the surface of the molten steel is covered with slag of 15 kg/steel-ton (amount per one ton of the molten steel) or more containing CaO and Al2O3 as a principal component. Then, an oxygen containing gas is blown onto the molten steel, and nitrogen in the slag is removed through the reaction between the oxygen and the slag, thereby to refining-produce a low-nitrogen steel having a nitrogen content of 20 ppm or less. More specifically, as shown in Expression (3), (N3−) shifted from the molten steel to the slag is then oxidized with the oxygen containing gas to be volatilized and removed. Further, oxygen blowing to the molten steel is blocked by slag, and reduction in the [Al] content of the molten steel is suppressed, thereby to accomplish nitrogen removal in the molten steel in accordance with Expression (2). In addition, a technique developed by improving the technique described in Japanese Unexamined Patent Application Publication No. 8-246024 described above is disclosed (see Japanese Unexamined Patent Application Publication No. 9-165615). According to this improved technique, after the surface of molten steel retained in a refinement vessel is covered with the slag, the oxygen containing gas is blown on the surface of the covering slag to an extent not causing direct contact with the molten steel. In addition, a nitrogen-removal flux in a powder form containing CaO, Al2O3, and CaC2 is directly blown into the molten steel, thereby to accomplish nitrogen removal. This technique expects the reaction represented as Expression (4) below.[N]+3/2(O2−)+3/2(CaC2)=(N3−)+3/2(CaO)+3[C]  (4)